The glass used for wearable equipment such as, for example, glasses with projector, a glasses-shaped or goggles-shaped display, a virtual reality and augmented reality display device, and a virtual image display device is required to have a high refractive index (for example, a refractive index nd of 1.68 or more) from the viewpoint of realization of a wide-angle and high-luminance/high-contrast image, enhancement of light guide properties, and easiness of processing of diffraction grating. In addition, thin plate formation of glass is demanded for the purpose of weight reduction. Moreover, a small-sized imaging glass lens with a wide imaging angle of view has been conventionally used in applications such as vehicle-mounted camera and robot's visual sensor, and a high refractive index is required of such an imaging glass lens so as to photograph a wider range with a smaller lens.
Furthermore, the wearable equipment above is typically configured by stacking a glass substrate serving as a waveguide so that a virtual image can be perceived with a comfortable and natural feeling by the human eye (see, for example, Patent Literature 1).